Capacitance gages have been used in a variety of vehicle environments for measuring the level of fuel in a tank. Capacitance gages have no moving parts and are therefore very reliable, especially in hostile environments where vibration and temperature extremes render mechanical level sensors useless. Capacitance gages allow for relatively simple compensation of various tank shapes, where linear changes in fluid level do not correspond to linear changes in fluid volume. Changes in dielectric constant in the medium separating the plates of the measuring capacitor will cause a change in measured capacity Air has a nominal dielectric constant equal to 1.0, and common oils or fluids, such as kerosine or gasoline have nominal dielectric constants of 2.0. Such fluids rising between two parallel plates will increase the net capacitance of the measuring cell as a function of fluid height. The fluid being measured may not vary in dielectric constant, or changes in measured capacitance may erroneously be attributed to the level changes.
Ethanol and methanol have dielectric constant of 24 and 31, respectively, and water has a dielectric constant of 78. Relatively small concentrations of these additives to gasoline will change the dielectric constant of the resulting mixture by a great amount.
There are several level sensing assemblies on the market which utilizes a level or measuring sensor capacitor and a reference capacitor which determine dielectric constant.
U.S. patent application Ser. No. 297,113, filed Jan. 13, 1989 in the name of one of the inventors, Peter A. Hochstein, named herein, discloses a capacitive liquid sensor structure and control circuit for sensing level. That control circuitry may be used in the subject invention, however, the structure of that sensor assembly is not used. The application discloses sensor assemblies which include a level or measuring capacitor and a reference capacitor. The level capacitor is mounted on the upper portion of a housing and the reference capacitor is mounted on a lower portion of the housing. Various designs are shown in the disclosure. Although it operates well, there is an operational range deficiency because after the level of liquid in the container decreases to the point below the level capacitor, the assembly will not properly indicate the amount of fuel in the tank. In other words, when the level of liquid reaches the reference capacitor below the level capacitor, the control circuitry may indicate empty even though the container remains partially filled.
Other such assemblies are disclosed in U.S. Pat. No. 3,921,451, issued Nov. 25, 1975 in the name of DiGiacomo. The application discloses the use of a level or measuring capacitor and a reference capacitor. The reference capacitor is located in the lower portion of the tank and the level sensing capacitor is located along the depth of the tank. Such an assembly allows for extensive measuring of the level of liquid, however, the reference capacitor will not always remain submerged, which is cause error in the indication of liquid level.
A third assembly is disclosed in U.S. Pat. No. 4,590,575, issued May 20, 1986 in the name of Emplit. The patent discloses a level sensing system which utilizes a reference capacitor and level or measuring capacitor. The capacitors include a probe which interacts with the walls of the tank or container to establish the capacitor. The problem with such an assembly is that the sizes of the tanks must be known since the capacitance is established between the probe and the tank walls. Such is not feasible since each tank may be of different size and of the manufacturing process, or may be made of a non-conductive material.